The present invention relates to an optical fiber used for relatively long-distance transmission, that is mainly applicable to super high-speed transmission or to multiplex transmissions with a high wavelength density.
Single mode fibers are familiar examples of conventional optical fibers used in high-speed transmission. These single mode fibers for high-speed transmission are usually formed of quartz glass, where the term as employed here shall mean quartz glass having silicon dioxide as the main component. In addition, the quartz glass forming the optical fiber core in this specification is understood to be quartz glass in which at least 50 wt % or more of the composition is silicon dioxide.
A single mode fiber of the simplest structure has a step-refractive index distribution. This step-index single mode fiber is designed with a cladding which is around and in contact with a core. The core has a uniform refractive index and the cladding has a lower refractive index than the core.
The electromagnetic field of a step-index single mode fiber can be determined by solving a Maxwell equation.
If the radius of the core in a cross-section of the optical fiber is designated as a, the refractive index of the core (peak refractive index) is designated as n1, and the refractive index of the cladding is designated as nclad then the core-cladding relative index difference of refraction (i.e., relative refractive index difference) xcex94 can be expressed by the following Equation (1)                                                         Δ              =                              xe2x80x83                            ⁢                                                (                                                            n                      1                      2                                        -                                          n                      clad                      2                                                        )                                /                                  (                                      2                    ⁢                                          n                      1                      2                                                        )                                                                                                        ≈                              xe2x80x83                            ⁢                                                (                                                            n                      1                                        -                                          n                      clad                                                        )                                /                                  n                  1                                            ≈                                                (                                                            n                      1                                        -                                          n                      clad                                                        )                                /                                  n                  clad                                                                                        Equation  (1)            
Setting the light wavelength to xcex, the normalized frequency V can be expressed by the following Equation (2).
V=(2xcfx80/xcex)an1(2xcex94)xe2x80x83xe2x80x83Equation (2)
Single mode conditions enabling only a single LP mode to be propagated are assured provided that this normalized frequency V is below a given set value.
The LP mode (i.e., linearly polarized mode) will now be explained.
The mode which propagates through the optical fiber core is referred to as the xe2x80x9cpropagating modexe2x80x9d and the mode which propagates through the clad is referred to as the xe2x80x9ccladding mode.xe2x80x9d The cladding mode radiates to the outside as it propagates over a specific distance, and becomes attenuated.
Strictly speaking, the propagating mode consists of modes that have a variety of directional components in the form of electromagnetic field vectors like TE, TM, HE, EH, etc. In a given approximation, or more specifically, under the condition in which the core-cladding relative refractive index difference is small, when perpendicular axes are placed in the fiber""s cross-sectional plane, it is possible to approximate the propagation state of the light using an LP mode which has an electromagnetic field vector in only one of the two perpendicular directions. In general, it is said that the relative refractive index difference between the core and the cladding is 1% or less. Provided that slight error is allowed, however, an approximation can be established in the case of a refractive index difference of up to 3%.
The correspondence between the LPmn mode and the strict field mode is as follows.
LP01 mode=HE11 mode
LP11 mode=TE01 mode, TM01 mode, HE21 mode
LP21 mode=EH11 mode, HE31 mode
LP02 mode=HE12 mode
In a step-index single mode fiber, it is known that when Vxe2x89xa62.405, only the lowest order mode (the fundamental mode, i.e., the LP01 mode) meets the single mode conditions for propagating through the core.
As may be understood from Equation (2) above, the disadvantage of this step-index single mode fiber is that, in order to fulfill single mode conditions for a given wavelength xcex, the product of core radius a (or core diameter 2a) and the square root of the relative refractive index difference xcex94xc2xdcannot be increased. In other words, in order to satisfy single mode conditions, the mode field diameter (MFD), which describes the region in which the mode is present, tends to become smaller in principle. When the MFD is small, however, it is not possible to satisfy the conditions for low-loss connection of plural optical fibers.
On the other hand, if an attempt is made to increase the MFD while maintaining the condition Vxe2x89xa62.405, it becomes necessary to expand core diameter 2a and thus decrease the relative refractive index difference xcex94.
When this type of design is executed, however, the refractive index difference is small, and the mode is large and spreads out from the core center. As a result, if only a slight bend (i.e., a microbend) is applied to in the fiber, loss readily occurs as the energy of the propagating mode passes through the cladding and is radiated to the outside.
Accordingly, as one countermeasure, rather than strictly maintaining the condition Vxe2x89xa62.405 shown in Equation (2), it is theoretically possible to set V so that a second order mode LP11 mode can be present.
In other words, if a design is provided that permits a value of about 3.0 for V, then there is strong containment of the electromagnetic field inside the core, even when setting the comparatively large MFD of the LP01 mode. For this reason, even if a slight bending is applied to the fiber, the bending loss does not become very large, so that transmission is possible.
Since the LP11 mode is only slightly contained within the core at this time, it does not propagate over long distances, but is attenuated quickly as it propagates over several to dozens of meters due to the large radiating losses from bending that the fiber incurs under the conditions in which it is actually employed. Thus, the LP11 mode does not effect transmission.
However, in a design in which two or more modes propagate in this way, the following problems may occur if the higher order mode does not quickly attenuate.
In general, when there are multiple modes propagating through an optical fiber, the individual modes do not have equivalent propagation speeds. For this reason, when the energy of an optical signal is distributed to a plurality of modes and simultaneously propagated in an optical fiber communications system, the individual modes will arrive at different times following propagation over a long distance, and the signal waveform following demodulation will be distorted. Accordingly, the effective result is that high-speed transmission is not carried out. In recent years, optical communications typically have been carried out at a transmission speed of several Gb/s or more per one wave in the propagation wavelength, with 10 Gb/s being reported on the level of practical applications, and 20xcx9c100 Gb/s being reported experimentally. However, the wavelength dispersion (or more simply, xe2x80x9cdispersionxe2x80x9d) in an optical fiber is determined based on the sum of the following two components. Namely, the first component is the material dispersion, which is determined by the material forming the fiber. The second component is the waveguide dispersion (i.e., structural dispersion), which is determined by the structure of the optical fiber""s refractive index distribution. In the 1.3xcx9c1.6 xcexcm wavelength region which is important for optical fiber communications, the material dispersion of a quartz type optical fiber tends to increase as the wavelength becomes longer. In the above-described typical step-index single mode fiber, the waveguide dispersion contribution is small, with material dispersion dominating. Thus, total dispersion, i.e., the sum of material dispersion and waveguide dispersion, becomes zero near 1.3 xcexcm.
The minimum loss wavelength of an optical fiber, particularly an optical fiber having quartz glass as the main component, occurs at around 1.55 xcexcm. The loss in a quartz optical fiber is mainly due to Rayleigh scattering, and becomes minimal in the 1.55 xcexcm band. Thus, in this wavelength band, a step-index single mode fiber in which V is 2.4xcx9c3.0 has a large dispersion and is not very suitable for high-speed transmission.
A dispersion shifted fiber is one in which the wavelength band where dispersion is zero has been shifted to the 1.55 xcexcm band in a single mode fiber consisting of quartz glass. In other words, the absolute value of waveguide dispersion, which is highly dependent on structure, is increased by changing the structure of the refractive index distribution, and the wavelength band at which the total dispersion, i.e., the sum of the material dispersion and the waveguide dispersion, becomes zero is shifted from the 1.3 xcexcm band. Material dispersion is determined by the material itself, and has very little dependence on the waveguide structure.
By making the dispersion in the 1.55 xcexcm band zero in this way, it is possible to carry out transmission with even less loss than in the 1.3 xcexcm band.
Specific values become as follows for example.
The material dispersion of regular quartz glass is roughly 17 ps/km/nm in the 1.55 xcexcm wavelength band. Thus, if the waveguide dispersion is approximately xe2x88x9217 ps/km/nm, this will cancel out material dispersion, so that dispersion can be rendered zero.
In order to increase the absolute value of the waveguide dispersion in this way, the following conditions must be satisfied.
(A) There must be a relatively large relative refractive index difference.
(B) There must be a relatively small core diameter, and the electromagnetic field distribution must have a relatively large spread with respect to the main components of the core.
Condition (A) can be met by designing the core-clad relative refractive index difference to be large.
Condition (B) is roughly synonymous with weak containment of light within the core. It is known in a dispersion shifted fiber that waveguide dispersion tends to increase in the region where xcex94(MFD)/xcex94xcex has a large value with respect to a xcexxe2x86x92xcex+xcex94xcex wavelength change. Thus, in order to increase the waveguide dispersion in a dispersion shifted fiber in accordance with condition (B), it is frequently the case that a design is executed in which the electromagnetic field greatly leaks out from the main part of the core.
However, when the electromagnetic field is large in this way, i.e., in a fiber having a large MFD, the mode greatly spreads out from the center of the core as described above. For this reason, if even a slight bend is applied to the fiber, the energy of the propagating mode is radiated to the outside, and loss readily occurs.
Thus, it is known to be extremely difficult to design a dispersion shifted fiber which simultaneously satisfies this bending loss sensitivity and a dispersion shift to the 1.55 xcexcm band.
Accompanying the advances being made in optical communications technology in recent years, a technique has been realized for carrying out long distance transmission while directly amplifying an optical signal using an optical amplifier. An erbium-doped fiber amplifier (EDFA) is employed as the aforementioned optical amplifier, with the amplified optical signal frequently having a power of 20-100 mW or more.
The 1.55 xcexcm band, which is the low loss region for a dispersion shifted fiber, has a certain wavelength width. In addition, the width of the region amplified by the EDFA has a wavelength width of 20 to 100 nm. A wavelength multiplexing transmission method was therefore realized in which a plurality of different light signals having 20 to 100 wavelengths are set within the 1.55 xcexcm band, and are transmitted in a single dispersion shifted fiber while simultaneously being amplified by the EDFA.
As a result of this type of technological progress, designing the fiber by widening the region (i.e., the effective core cross sectional area) where the light is present in the optical fiber, i.e., widening the MFD, has a significance beyond just the goals of connecting fibers with low loss, increasing the absolute value of waveguide dispersion in an dispersion shifted fiber, etc. Namely, we are referring to the problem of nonlinear effects.
A problem occurs in long-distance transmission in which, when a sufficiently amplified large power optical signal is transmitted over a long distance, the signal waveform of the optical signal becomes distorted due to the influence of nonlinear effects. This problem occurs irrespective of whether or not wavelength multiplex transmission is present.
Self-phase modulation, four-wave mixing (i.e., FWM) and the like may be cited as specific examples of nonlinear effects.
Self-phase modulation is one of the third order nonlinear phenomena that cause refractive index changes in a substance depending on the light intensity. In self-phase modulation, the phase of the optical pulse itself that is propagating through the substance changes abruptly during a short period of time.
In long-distance transmissions, even when transmitting one wave for example, a phenomenon occurs when the peak power of an optical signal is strong in which the glass has a different refractive index at peak positions where power is strongest and valley positions where power is weakest. As a result, localized changes in the instantaneous frequency of the light occur.
Since the change in the instantaneous frequency becomes larger as modulation becomes faster, this becomes linked to the dispersion in the optical fiber and causes a large waveform distortion. Thus, self-phase modulation in a long distance multiplex transmission is an effect that is better referred to as an interaction between self-phase modulation and dispersion in a dispersion shifted fiber.
FWM is also one of the third order nonlinear phenomena. An unnecessary fourth light is generated by three incidenting lights, and the waves of the four frequencies interact to cause an effect on wavelength multiplex communications. It is possible to conceive of an extremely large number of four-wave combinations as the number of wavelength multiplexes increases, so that many of these mutually interact to cause a deterioration in the quality of communication.
The generation efficiency of the unnecessary light (waves) due to FWM can be obtained as an approximation from the following Equation (3).
xcex7=(xcex12xc2x7n22)/(D2xc2x7Aeff2)xe2x80x83xe2x80x83Equation (3)
In this equation, xcex1 is the optical fiber loss coefficient (where the units are dB/km, for example), n2 is the nonlinear refractive index of the optical fiber glass, D is the dispersion of the optical fiber, and Aeff is the effective core cross-sectional area of the optical fiber.
Aeff in Equation (3) can be obtained from the following Equation (4) in the case where the electromagnetic field distribution of the mode propagating through the core is Gaussian.
Aeff=xcfx80xc2x7MFD2/4xe2x80x83xe2x80x83Equation (4)
However, as shown in the following Equation (5), Aeff is actually calculated by taking the integral of the electromagnetic field distribution of the light in the core.                               A          eff                =                                            {                                                ∫                                      -                    ∞                                    ∞                                ⁢                                                      ∫                                          -                      ∞                                        ∞                                    ⁢                                                                                    "LeftBracketingBar"                                                  E                          ⁡                                                      (                                                          x                              ,                              y                                                        )                                                                          "RightBracketingBar"                                            2                                        ⁢                                          ⅆ                      x                                        ⁢                                          ⅆ                      y                                                                                  }                        2                                              ∫                              -                ∞                            ∞                        ⁢                                          ∫                                  -                  ∞                                ∞                            ⁢                                                                    "LeftBracketingBar"                                          E                      ⁡                                              (                                                  x                          ,                          y                                                )                                                              "RightBracketingBar"                                    4                                ⁢                                  ⅆ                  x                                ⁢                                  ⅆ                  y                                                                                        Equation  (5)            
As may be understood from Equation (3), as the dispersion of the optical fiber approaches zero, the generation efficiency becomes very large. Thus, from the perspective of high-speed transmission, it is desirable that the dispersion be as small a value as possible. However, on the other hand, a dispersion value that is too small is inconvenient from the perspective of nonlinear effects.
Further, it is preferable that Aeff be large. Accordingly, designing the MFD to be large as described above has a significant effect with respect to decreasing nonlinear effects.
In view of the preceding background, recent dispersion shifted fibers have had to satisfy the following conditions.
(A) The absolute value of the dispersion must be small in the employed wavelength band, but may not be completely zero, and should have a value that is deviated to a certain extent (referred to as a xe2x80x9cnon-zero dispersion shifted fiberxe2x80x9d).
(B) Aeff must be large.
(C) Loss must be low. This is satisfied to some extent if a quartz optical fiber is employed. Specifically, it is preferable that the loss in the 1.55 xcexcm band be less than about 0.23 dB/km.
(D) Bending loss sensitivity must be small. This occasionally may be inconsistent with the aforementioned second requirement that Aeff be large.
The present invention was conceived in view of the above-described circumstances and has as its objective the provision of an optical fiber having (A) a relatively large Aeff, (B) low loss, (C) an absolute value for dispersion of around several ps/km/nm in the 1.55 xcexcm band, and (D) a relatively small bending loss sensitivity in the environment in which the optical fiber is employed.
When employed for wavelength multiplex transmission, it is preferable that (E) broad-band transmission be possible in long distance transmission over several km or more.
In order to resolve the above described problems, the present invention employs the following means.
Namely, the invention according to claim 1 is a multimode optical fiber with a higher order mode removing function, wherein at least three or more linearly polarized wave modes can exist as propagating modes when an optical signal incidents, characterized in that these propagating modes include the lowest mode and second or higher modes, and the difference between the propagation constants of the lowest order mode and the second order mode is 2-fold or more than the difference between the propagation constants of adjacent modes that are second or higher modes.
The invention according to claim 2 is a multimode optical fiber with a higher order mode removing function according to claim 1, characterized in that the difference in the normalized propagation constant between adjacent modes in the second or higher order modes and a cladding mode is 0.25 or less.
The invention according to claim 3 is a multimode optical fiber with a higher order mode removing function according to claim 1, characterized in that said multimode optical fiber is provided with a core and a cladding which is around and in contact with this core, the core consists of two or more layers provided in a concentric form, and is equipped with a maximum refractive index layer that has the highest refractive index and is provided near the center of the core, and an intermediate layer that has a refractive index that is lower than that of the maximum refractive index layer and is around and in contact with the maximum refractive index layer.
The invention according to claim 4 is a multimode optical fiber with a higher order mode removing function according to claim 3, characterized in that the maximum value of the relative refractive index based on the cladding of the intermediate layer is 5xcx9c90% of the relative refractive index based on the cladding of the maximum refractive index layer.
The invention according to claim 5 is a multimode optical fiber with a higher order mode removing function according to claim 3, characterized in that the difference in the relative refractive index difference based on the refractive index of the cladding in the maximum refractive index layer is in the range of 0.65xcx9c1.5%.
The invention according to claim 6 is a multimode optical fiber with a higher order mode removing function according to claim 3, characterized in that the outer diameter of the core is 3xcx9c8-fold greater than the outer diameter of the maximum refractive index layer.
The invention according to claim 7 is a multimode optical fiber with a higher order mode removing function according to claim 3, characterized in that the outer diameter of the core is 3xcx9c5.5-fold greater than the outer diameter of the maximum refractive index layer.
The invention according to claim 8 is a multimode optical fiber with a higher order mode removing function according to claim 3, characterized in that the intermediate layer consists of one layer or two or more layers having different refractive indices, and, when the refractive indices of the layers forming the intermediate layer are designated nl1, nl2, . . . , nli (i=2, 3, . . . ) starting from the inside, nl1 greater than nli, and the cladding is provided with a refractive index lower than nl1.
The invention according to claim 9 is a multimode optical fiber with a higher order mode removing function according to claim 8, characterized in that the maximum value of the relative refractive index based on the cladding of the intermediate layer is 5xcx9c50% of the relative refractive index based on the cladding of the maximum refractive index layer.
The invention according to claim 10 is a multimode optical fiber with a higher order mode removing function according to claim 3, characterized in that the intermediate layer consists of two or more layers having different refractive indices, and, with respect to the layers forming the intermediate layer, when the refractive index of the layer adjacent to the maximum refractive index layer is designated nl1 and the maximum refractive index of these layers is designated nlmax, then nlmax greater than nl1; and the cladding is provided with a refractive index lower than nlmax.
The invention according to claim 11 is a multimode optical fiber with a higher order mode removing function according to claim 10, characterized in that the maximum value of the relative refractive index based on the cladding of the intermediate layer is 15xcx9c90% of the relative refractive index based on the cladding of the maximum refractive index layer.
The invention according to claim 12 is a multimode optical fiber with a higher order mode removing function according to claim 1, characterized in that modes other than the lowest order mode are attenuated during the time that an incidented light signal is propagating a maximum of 4 km, and do not essentially contribute to data transmission.
The invention according to claim 13 is a multimode optical fiber with a higher order mode removing function according to claim 1, characterized in that the effective core cross sectional area in the 1.55 xcexcm band is 50 xcexcm2 or more, the absolute value of dispersion in 1.55 xcexcm band is 10 ps/km/nm or less, and the main component is quartz glass.
The invention according to claim 14 is a multimode optical fiber with a higher order mode removing function according to claim 13, characterized in that the effective core cross sectional area in the 1.55 xcexcm band is 70 xcexcm2 or more.
The invention according to claim 15 is a multimode optical fiber with a higher order mode removing function according to claim 14, characterized in that the absolute value of dispersion in 1.55 xcexcm band is 5 ps/km/nm or less.
The invention according to claim 16 is a multimode optical fiber with a higher order mode removing function according to claim 1, characterized in that the number of propagating modes is 3xcx9c6.
The invention according to claim 17 is a multimode optical fiber with a higher order mode removing function according to claim 1, characterized in that in the employed wavelength band, dispersion of the lowest order mode becomes zero at a wavelength longer than 1.5 xcexcm.
The invention according to claim 18 is a multimode optical fiber with a higher order mode removing function according to claim 1, characterized in that the uniform bending loss for a diameter of 20 mm in the employed wavelength band is 30 dB/m or less.
The invention according to claim 19 is a multimode optical fiber with a higher order mode removing function according to claim 1, characterized in that the uniform bending loss for a diameter of 20 mm in the employed wavelength band is 10 dB/m or less.
The invention according to claim 20 is a multimode optical fiber with a higher order mode removing function according to claim 1, characterized in that in the employed wavelength band, dispersion of the lowest order mode becomes zero at a wavelength shorter than 1.5 xcexcm.
The present invention""s multimode optical fiber with a higher order mode removing function is not particularly restricted. However, since the present invention intends to provide a design suitable to long-distance transmission, a presumption is made that the invention will be employed in the 1.55 xcexcm band as a general rule. This 1.55 xcexcm band has a wavelength range of 1490xcx9c1620 nm.
In addition, while the effective core cross sectional area, bending loss and the like are not particularly restricted, as a general rule, these are values measured in the employed wavelength band of 1.55 xcexcm.